In recent years, it has been recognized that bubbles of a size not larger than 1 mm called “micro-bubbles” have various excellent properties, and such micro-bubbles have been drawing a lot of attention. The micro-bubbles are known to exhibit an extremely excellent effect in activation of fishes, domestic animals and plants, and in purification of wastewater, for example.
By the way, there is known a micro-bubble generating device constructed to generate such micro-bubbles and mix the micro-bubbles into a liquid, as disclosed in JP-A-2006-159187, for instance. This micro-bubble generating device has a stationary type mixer provided with a cylindrical body consisting of an upstream screw portion and a downstream cutter portion. Two phases of fluids consisting of a gas and a liquid introduced through inlet tubes into the stationary type mixer are brought into a turbulent state while being subjected to a rotating force and a large torsional force of helical blades of the screw portion, and are then brought into contact with a plurality of protrusions formed on an inner circumferential surface of the cylindrical body, whereby turbulent flows of the two gas-liquid phases are produced to promote resolution of the gas in the liquid.
In such a conventional device, the screw portion of the stationary type mixer has the helical blades disposed within the cylindrical body, and the cutter portion has the plurality of protrusions formed on the inner circumferential surface of the cylindrical body, so that the device is comparatively complicated in construction, and has an accordingly high cost of manufacture. Further, the bubbles generated and mixed into the liquid cannot be really qualified as the micro-bubbles.
Recently, it has been considered to utilize a micro-bubble generating device as a device for supplying oxygen or other gas into water in appreciative-fish or live-fish preservation water tanks, or as ventilation means for drainage treatment tanks, fermentation tanks or culture tanks. Specific constructions of such a micro-bubble generating device are proposed in Japanese Patent No. 3806008 and JP-A-2007-268390.
Under the situation described above, the present applicant proposed a dispersing device (micro-bubble generating cylindrical body) to be used for the micro-bubble generating device to generate the micro-bubbles, as disclosed in Japanese Utility Model Registration No. 3130562. This dispersing device has a cylindrical body which is formed of a porous material and an outer circumferential surface of which is covered by a hydrophilic unwoven fabric or the like. The porous material used for this dispersing device is preferably a gas-permeable film (air-permeable film) that is a high molecular resin film with crazes generated therein.
The dispersing device proposed by the present applicant is immersed in water, and its inner space is supplied with compressed air, oxygen or other gas. The gas changes into micro-bubbles as a result of its permeation through the gas-permeable film or other porous material, and the unwoven fabric, and the micro-bubbles emerging from the surface of the unwoven fabric are dispersed into the water. In this dispersing device wherein the unwoven fabric has a hydrophilic property, the bubbles which permeate through the gas-permeable film and flow into the unwoven fabric are broken down into smaller pieces by the water existing within the unwoven fabric, so that the size of the micro-bubbles generated can be further reduced.
The present inventors have made an analysis of such a dispersing device, in an effort to improve its utility, and have found a risk of reduction of the amount of generation of the micro-bubbles due to clogging or plugging of pores within the unwoven fabric, with foreign matters such as microorganisms and suspended products in the water, which adhere to the surface of or flow into the unwoven fabric during a long period of time of use in the water.